In the technique of laser enrichment utilizing isotopically selective ionization of a vapor flow of metallic uranium as shown in U.S. Pat. Nos. 3,772,519 and 3,939,354, one or more evacuated chambers are utilized. The chambers are typically evacuated to a pressure of a small fraction of a millitorr. Uranium vapor is generated within the atmosphere preferably by electron beam vaporization techniques with the resulting vapor expansion directed toward a region of collection and accelerating electrodes where the desired U-235 isotope is separated by photoionization and crossed-field MHD acceleration techniques. While the vapor source temperature is in excess of 1000.degree. C, the chamber within which the apparatus is placed is typically cooled to protect such elements as elastomeric vacuum seals located throughout the chamber walls. Radiative cooling of the electrode and collection plate structure of the ion extractor to the far colder walls will result also. The waste vapor, that depleted of the U-235 isotope, is collected on a set of upper surfaces which also are cooled by radiative heat loss to the chamber walls. In addition, the chamber walls themselves due to cooling are at a reduced temperature.
The uranium vapor generated in the chamber will collect to a greater or lesser degree upon all these surfaces. At the various plate temperatures assumed under these conditions, which may be all or nearly all below 400.degree. or 450.degree. C, the uranium tends to collect as a mass of relatively low density, substantially below the theoretical maximum density of the material. Such a deposition has a high surface area to volume ratio which can create a condition of pyrophoricity, that is self combustion, of the uranium deposits when the chambers are exposed to the atmosphere. This may occur when the chambers are opened for servicing or removal of the deposits. Among other problems, the pyrophoricity of the deposits results in a loss of the enriched uranium when exposed to the atmosphere. While it is possible to open the chambers in an inert atmosphere for service and for deposit collection, it is preferable to be able to operate upon the open chambers in the normal atmospheric environment.